Sun in a Bottle - Charles Seife [49]
This feedback between electric fields and magnetic fields is just one of many effects that make plasmas hard to predict. Another has to do with the density of the plasma. Electric currents behave differently in plasmas of different densities and pressures. A current passing through a cloud of plasma alters the shape and density of the cloud—a pinch compresses the shape and increases the density—but this change alters the nature of the current passing through the cloud. This changes the shape and density of the cloud, which alters the current, which alters the shape and density of the cloud, and so on. Yet another issue had to do with the very makeup of the plasma. Scientists had been trying to ignore the fact that a plasma is not a nice, homogeneous substance made of a single kind of particle. A plasma is made of very heavy positively charged particles (the nuclei) and very light negatively charged particles (the electrons stripped from the atoms). These two kinds of particles have different properties and behave differently even when they are at the same temperatures, at the same pressures, and subjected to the same electromagnetic fields. Physicists discovered that when they tried to heat a plasma, unless they were very careful they would pour most of the energy into the light (and easy to accelerate) electrons, leaving the heavy nuclei cold, unheated, and slow. This was really bad news. The whole point of heating the hydrogen plasmas was to heat up the nuclei so that they were moving fast enough to fuse; hot electrons and cold nuclei were all but worthless. Unless scientists could compel the hot electrons to share their energy with the nuclei, there would be no hope of fusion. For all these reasons—and more—plasmas were very hard to work with. Even before a plasma gets hot and dense enough to ignite, it is a fiendishly complex brew.
The brew did not behave the way scientists expected it to. It seemed to have a mind of its own, thwarting all attempts to keep it under control. Pinch it or squeeze it or even try to keep it confined in a magnetic trap and it writhed around and ruffled itself in instability after instability. Physicists built bigger and more expensive machines to wrestle the instabilities into submission, but they were failing. As the machines started costing millions and tens of millions of dollars, the scientists were no closer to building a fusion reactor than before; they were just uncovering more and more subtle ways that the plasma fought their will.
Lyman Spitzer’s Stellarator, for one, was mysteriously losing the particles in its plasmas. High-temperature particles move very quickly and are inherently hard to constrain. It was no surprise then that hot plasma particles in a crude magnetic bottle would rapidly spiral out of control and slam into the walls of the vessel, and the higher the temperature, the faster the particles were lost. On the other hand, increasing the magnetic field strength—strengthening the bars of the magnetic cage containing the plasma—should have rapidly brought this problem under control. That was the theory, anyhow. The researchers thought that if they doubled the magnetic field, they should cut the loss rate by a factor of four. If this theory was right, it would be fairly simple to get particle losses under control merely by cranking up the strength of the magnets surrounding the plasma. Relatively weak magnetic fields would suffice to confine even very hot plasmas.
Nature wasn’t quite so kind to the Stellarator. As the scientists turned up the magnetic fields, they were surprised to discover that particles still zoomed out of control very quickly.